Gypsum boards are widely used in various kinds of buildings as architectural interior finish materials, because of their advantageous fire-resisting or fire-protecting ability, sound insulation performance, workability, cost performance and so on. The gypsum boards are known as boards having a gypsum core covered with sheets of paper for gypsum board liner. In general, the gypsum boards are produced by a continuous slurry pouring and casting process. This casting process comprises the following steps:
(i) a step of admixing calcined gypsum, adhesive auxiliary agent, set accelerator, foam for reducing a weight, admixtures, additives or the like, with water in a mixer, thereby preparing calcined gypsum slurry (referred to as “slurry” hereinafter);
(ii) a step of pouring the slurry thus prepared in the mixer, into an area between upper and lower sheets of paper for gypsum board liner and forming them in a continuous web-like formation having a configuration of board; and
(iii) a step of severing solidified continuous web-like formation, drying it forcibly and thereafter, cutting it to be a product size.
Normally, a thin type of circular mixer is used as a mixer for preparing the slurry. This type of mixer comprises a flattened circular housing and a rotary disc rotatably provided in the housing. A plurality of feeding ports for components to be mixed are located in a center region of an upper plate of the housing, and a slurry outlet port for delivering mixture (slurry) from the mixer is provided in a periphery of the housing. A rotary shaft is connected to the disc to rotate the disc. The upper plate of the housing is provided with a plurality of upper pins (stationary pins) depending therefrom to the vicinity of the disc. The disc is provided with lower pins (movable pins) which are vertically fixed on the disc and which extend to the vicinity of the upper plate. The upper and lower pins are arranged in radially alternate positions. The components to be mixed are supplied on the disc through the respective feeding ports, and are agitated and mixed while being moved radially outward on the disc under an action of centrifugal force, and then, delivered out of the slurry outlet port. The mixer with this arrangement is called a pin type of mixer, which is disclosed in, e.g., US Patent Publication No. 3,459,620.
In general, foam is fed into the mixer for regulating the specific gravity of gypsum board. The importance has been attached to proper mixing of the foam in the slurry, especially in a method of producing lightweight gypsum boards. The following publications are exemplified as prior art intended to achieve effective mixing of foam into slurry:
(1) Japanese Patent Laid-Open Publication No. 8-25342 (Mixer and Mixing Method)
(2) Japanese Patent Laid-Open Publication No. 11-501002 (Method for Preparing Foamed Gypsum Product)
(3) U.S. Patent Publication No. 6,494,609 (Slurry Mixer Outlet)
Further, a relatively large stiff solidified gypsum (residuum) may be fed to the gypsum board liner paper from the mixer. The following publication is exemplified as prior art which is intended to prevent a continuous operation of gypsum board producing apparatus from being unexpectedly interrupted owing to this situation:
(4) Japanese Patent Laid-Open Publication No. 2000-6137 (Mixer and Method of Producing Gypsum Boards with the Mixer)
(1) Japanese Patent Laid-Open Publication No. 8-25342 discloses a mixer and mixing method, wherein quantities of gypsum slurry, which considerably differ in the specific gravity, can be respectively fed to a gypsum board liner paper and so forth by a single mixer. The mixer disclosed in JP 8-25342 comprises a partition wall depending from a periphery of a housing upper plate down to a level close to a rotary disc, in such a way that the inside area of the mixer is divided into two areas (an inward area and a peripheral zone) by the partition wall. In the peripheral zone of the upper plate, there is disposed a foam feeding part, which feeds foam for regulating the volume of slurry for a core of gypsum board (slurry with a low specific gravity). A plurality of slurry fractionation ports are disposed on an annular wall of the housing or the peripheral zone of a housing lower plate, upstream of the foam feeding part as seen in the direction of rotation. Further, a slurry outlet port is disposed on the annular wall or the peripheral zone of the lower plate, downstream of the foam feeding part in the direction of rotation. The fractionation ports and the slurry outlet port can respectively discharge quantities of slurry which differ in the content of foam from each other, and therefore, the quantities of slurry differing in their specific gravity can be supplied to predetermined portions of the gypsum board producing apparatus, respectively. JP 8-25342 also discloses an arrangement in that a second foam feeding section (feeding conduit) is positioned on an upper end portion of a slurry delivery conduit. The slurry delivery conduit is connected with the slurry outlet port by means of a hollow connector section, and it is also called as “vertical chute” or “canister”. The slurry in the slurry delivery conduit (slurry for the core) is fed with the foam. With such an arrangement, quantities of slurry differing in their specific gravity can be prepared more efficiently by a single mixer, and the consumption of foaming agent can be reduced.
In accordance with this arrangement of the mixer, it would be possible to supply the gypsum board liner paper with high-quality slurry uniformly mixed with foam, so far as the speed of production (the production rate) of gypsum boards is restricted to be at a relatively low speed so as not to increase the flow rate of slurry. However, it has been found that, as speeding-up of production of gypsum boards is made and the flow rate of slurry is increased, a phenomenon occurs in which the foam and the slurry are not uniformly mixed. That is, if speeding up of production of gypsum boards is made, a desirable mixing condition of the slurry and the foam can not be ensured by the mixer disclosed in JP 8-25342. For instance, phenomenon such as gas pocket of a relatively large size confined to an interface between the gypsum board core and the liner paper covering the core is apt to occur (this defectiveness is known as “bulging”). This is considered to result from separation of slurry and the foam, which is caused in the slurry delivery conduit by effects of the vortex therein, centrifugal force and differing of specific gravity, as described in U.S. Pat. No. 6,494,609.
(2) In Japanese Patent Laid-Open Publication No. 11-501002, an arrangement of mixer is disclosed, in which an insertion point of aqueous foam for regulating slurry volume is properly positioned so as to minimize destruction of foam during mixing. In this mixer, the point of insertion is located, for instance, on an upper plate or an annular peripheral wall in position closer to a slurry discharge outlet than the location of an inlet for calcined gypsum, or located on a slurry discharge conduit connected to the slurry discharge outlet. Further, the mixer in the JP 11-501002 fractionates the slurry for the edge from an outlet different from the slurry discharge outlet, similarly to the mixer disclosed in JP 8-25342 as set forth above, so that the foam for regulating the slurry volume can be fed only to the slurry for core.
However, the mixer in JP 11-501002 does not have a structure corresponding to the slurry delivery conduit, and the slurry for core is directly discharged to the gypsum board liner paper through the discharge conduit. That is, the mixer in JP 11-501002 is so arranged as to immediately discharge the slurry in the mixer to the gypsum board liner paper through the discharge conduit attached to the annular wall of the mixer. For this reason, the foam fed from the foam feeding port on the discharge conduit or its vicinity can not be sufficiently mixed nor dispersed in the slurry when the flow rate of slurry is increased. Therefore, the mixer of this arrangement can run into difficulties when being applied to the speeding up of production rate of gypsum boards.
(3) A centrifugal mixer disclosed in U.S. Patent Publication No. 6,494,609 is provided with an outlet in a tangential direction on an annular wall. An elongated slurry conduit, which is in communication with the outlet opening to an inner area of the mixer, is connected with the mixer. The conduit has a discharge spout for discharging the slurry to a gypsum board forming area. The conduit is provided with a restrictor for creating back pressure so that a slurry filling condition is kept in the mixer by the back pressure. A reducer is provided at the discharge spout of the conduit, so that a slurry discharge pressure is reduced. According to such an arrangement of the mixer, the slurry in the conduit flows in a generally streamline state in the slurry delivery route between the slurry outlet and the discharge spout. The mixer disclosed in U.S. Pat. No. 6,494,609 is intended to provide a mixer for gypsum slurry which does not require use of a canister, taking into account the following points:
{circle around (1)} In a conventional mixer with canister (corresponding to the aforementioned “slurry delivery conduit” ), a vortex is caused in the canister, so that an empty air space is created in the canister;
{circle around (2)} Since such an air space is formed, solidified gypsum (residuum) is produced in the canister, and this results in clogging of the slurry supply passage;
{circle around (3)} With formation of vortex flow and generation of centrifugal force in the canister, the slurry is pushed against an inner wall surface of the canister. However, the foam tends to stay in a center area of the canister. Therefore, a portion of slurry having a high density unfavorably separates from the foam with a relatively low density (It follows that the canister takes a disadvantageous action from an aspect of uniform mixing of the slurry and the foam.)
In the mixer as disclosed in U.S. Pat. No. 6,494,609, the slurry is conditioned in a streamline flow state in the slurry conduit, and the foam is mixed into the slurry in such a condition. Therefore, the foam and the slurry might be able to be uniformly mixed. From this point of view, the mixer of U.S. Pat. No. 6,494,609 might be applicable to speeding up of gypsum board production. However, an unstable mixing condition of the foam and the slurry may be caused also in this arrangement of the mixer, when a production rate of gypsum boards is raised and the flow rate of slurry in the slurry conduit is increased. Further, as the elongated conduit is used for streamline flow of the slurry, the slurry tends to be adhered to the conduit and a solidified gypsum (residuum) is apt to be produced in the conduit. The growing solid gypsum (residuum) is finally discharged on the gypsum board liner paper together with the slurry. Such a solidified gypsum (residuum) results in a problem of shearing of gypsum board liner paper, which may cause downtime of gypsum board producing process. Therefore, this is an obstacle to continuous operation of the gypsum board producing apparatus. Accordingly, it is necessary for an operator to squeeze the conduit and/or the spout periodically, such as every 15 minutes, in order to prevent adhesion of slurry or growth of the solid gypsum in the conduit, as described in U.S. Pat. No. 6,494,609.
(4) As regards an enlarged and stiff residuum of slurry which may cause a problem of shearing the liner paper, Japanese Patent Laid-Open Publication No. 2000-6137 discloses an arrangement in that such a residuum is restricted in being discharged from the mixer, whereby interruption of continuous operation of the gypsum board producing apparatus is avoidable and gypsum boards can be stably produced. An attachment having openings for screening is located at a slurry outlet port of the mixer. The openings effect filtering of a stiff residuum which has a larger size than the size of opening, and prevent such a residuum from being discharged on the gypsum board liner paper. In this mixer, feeding conduits for powder materials, liquid (water) and foam are connected to an upper plate of the mixer in its inward area, so that the foam introduced into the mixer is fully agitated and mixed with the slurry within the mixer.
The mixer as described in JP 2000-6137 comprises the foam feeding port located on the upper plate in the inward area of the mixer, and therefore, the foam of the slurry is subjected to sufficient agitating action of the mixer so that uniform mixing of the foam and the slurry is obtained. However, this arrangement results in destruction of a relatively large quantity of foam subjected to a strong agitating action, and therefore, the quantity of foaming agent has to be increased, in correspondence with the amount of broken foam. Accordingly, the consumption of foaming agent is increased. As the production rate of gypsum boards is increased, this tendency becomes remarkable, and therefore, this is disadvantageous in cost saving of production.
Thus, when an attempt is made for speeding up of the gypsum board production line, the conventional mixer encounters at least one of the problems in uniform mixing of foam, stable supply of slurry in a high flow rate, and consumption of foaming agent.
It is an object of the present invention to provide a mixer and a mixing method for producing gypsum boards which are adaptable to speeding up in the continuous slurry pouring and casting type of gypsum board production line, which enable stable supply of a high flow rate of slurry uniformly mixed with the foam, and which enable reduction in the consumption of foam to be fed to the slurry.
It is another object of the present invention to provide a continuous slurry pouring and casting type of method for producing gypsum boards, which enables reduction in the consumption of foaming agent and speeding up of production, thereby improving productivity.